- #36
Grinkle
Gold Member
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AdrianMay said:That original angular momentum should decay by viscous friction.
It does not happen that way at least for water, or if it does, the decay does not go all the way to zero. If one experiments with fluids of varying viscosity, one might see the effects your intuition tells you should be present, it does seem to me that viscosity should play a big role in the swirling phenomena.
Edit:
Gravity keeps it going, there is no other force acting on the draining fluid to work against any friction that might be present.
You claim the equilibrium point for all forces acting on the fluid is zero angular momentum as though its obvious it cannot reasonably be any other number than zero. To me this is not at all obvious from inspection, and moreover it does not seem to happen that angular velocity decays to zero in practice.
Edit 2:
Think of a marble swirling down a funnel. It swirls faster and faster as it goes down the funnel. There is viscous friction (wind resistance) acting against it, but the dissipative force is very obviously not sufficient to cause the marble to stop swirling and roll straight down the side of the funnel. This is an example of a system that does have dissipative forces but these forces are not sufficient to bring angular momentum to zero. It not sufficient to simply note that dissipative forces exist in a system to establish that the equilibrium point of the system is zero for whatever state one is talking about (angular momentum in this case).
One can blow a fan against the marble and cause it to stop swirling, but that would be a different system.
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